Delayed automatic volume control



Sept.. 5, 1939. R. B. DOME DELAYED `UTOMATC VOLUME CONTROL Filed M ay 24, 1938 2 'Sheets-Sheet l Inventor: 'Robert B. Dome,

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Sept. 5, 1939. R, B, DOME I 2,172,160

DELAYED AUTOMATIC VOLUME coNTRoL Filed May 24, 1958' 2 sheets-Sheet 2 IhveboT-i Robert. B. Dome,

Patented Sept. 5, 1939 UNTED STATES enr OFMQE Robert B. Dome, Bridgeport, Conn., assigner to General Electric Company, a corporation of New York Application May 24, 1938, Serial No. 209,693

12 Claims.

My invention relates to an automatic volume control for signalling apparatus, and more particularly to an improved and simplified automatic volume control circuit of the delayed type.

To overcome the conditions of fading encountered in the'use of radio receivers operating over long distances, it has become customary to provide somey form of automatic volume control means for the receiver. This control operates to maintain the output of the receiver substantially constant over a large range of received signal intensities. As the signal decreases in strength, the amplification of one or more of the amplifier stages included in the receiver must be increased under action of the automatic volume control means. A condition of increasing signal strength requires that the amplification of the receiver be decreased. Ordinarily, however, it is desirable that the amplification be not decreased by the control until the signal strength is above a predetermined value; i. e., that the automatic volume control action be delayed until the signal attains a predetermined intensity.

One object of my invention is to provide improved means for obtaining this delayed automatic volume control action.

It is a further object of my invention to provide a system for producing this delayed type of automatic volume control action and one wherein` the delayed operation is accomplished in a simple and reliable manner and by the use of a minimum of circuit elements.

An additional object of my invention is to provide an arrangement for providing delayed automatic volume control action wherein a grid element of an electron discharge device included in the radio frequency stages of amplification operates to provide the delayed action of the automatic volume control circuit; To this end, my invention contemplates that the suppressor grid of an electron discharge device shall perform the dual function of providing a delayed action of the automatic volume control whenever received signals are below a predetermined signal strength, and shall operate as a suppressor grid for signal oscillations of any strength.

The novel features which I believe to be characteristic ofl my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description 'taken in connection with the accompanying drawings in which Fig. 1 illustrates (Cl. Z50-20) an embodiment of my invention, Fig. 2 illustrates certain characteristics pertaining to its operation, and Figs. 3, 4, and 5 illustrate modifications of my invention.

Referring to Fig. l of the drawings, my invention is illustrated as embodied in a signalling apparatus having a radio frequency amplifier, represented conventionally by the rectangle l, connected to a source of signal oscillations comprised by the antenna ground system il, l2. Theradio frequency amplifier lil may comprise as many stages of amplification as desired, but is here illustrated as being provided with at least two tandem connected stages each of which includes an electron discharge device I3, M. Included in` the radio frequency amplifier, represented by the rectangle l0, may be a converter stage for changing the signal oscillations to oscillations of an intermediate frequency together with one or more stages of amplification for amplifying the oscillations of intermediate frequency. The output of the radio frequency amplifier Ill is coupled through an intermediate frequency transformer i5 to the control grid I6 of an electron discharge device il. The transformer i5 has a primary i8 and a secondary i9 respectively tuned by the condensers 2li a'nd 2l to the frequency of the intermediate frequency oscillations. The electron discharge device il has a screen-grid 22 connected, as indicated, to a source 2@ of positive potential and has an anode 2d connected through the primary 25 of an intermediate frequency transformer 26 to the positive terminal of a source of anode potential, not shown. The intermediate frequency transformer 26 has a secondary winding 2l, the transformer windings 25 and 2l' being respectively tuned by the condensers 28 and 29 to the frequency of the oscillations of intermediate frequency.

The secondary winding 27 of the intermediate frequency transformer 26 is connected to the anode 30 and through a load resistor 3| to the cathode 32 of an electron discharge diode rectifier 33. The diode rectifier 33 rectiiies the signal oscillations and the modulation components of the signal oscillations appear as a unidirectional potential across the resistor 3i The audio frequency signal voltage, which appears on resistance 3i, is supplied through a condenser 34 to an audio frequency amplifier, represented conventionally by the rectangle 35, where the audio frequency oscillations are amplified for reproduction by a loud speaker 36 or other translating device.A

The receiver is provided with an automatic control circuit which operates to maintain the output of the receiver substantially constant over a wide range of variations in the strength of received signal oscillations. This is accomplished by supplying the unidirectional potential which appears across the resistor 3| through a ripple lter comprised by the resistors 38 and 40 and the condenser 39 to the control grids 4I and 42 of the respective electron discharge devices I3, I4. It will be apparent that increasing values of signal strength cause the ungrounded end of the resistor 3| to become increasingly negative. Since the potential across the resistor 3| is supplied to the grids 4I, 42 of the respective discharge devices I3, I4, increasing values of potential appearing across the resistor 3| cause the control grids 4I, 42 of the respective devices I3, I4 to be biased to an increasingly negative potential thereby to reduce the output of the radio frequency amplifier Il). In like manner, decreasing values of signal strength cause corresponding increases in the biasing potential applied to the control grids 4I, 42, of the respective discharge devices I3, I4, and the output of the radio frequency amplifier I thereupon increases.

A delay in the application of control potential by the automatic volume control circuit 31 to the control grids 4I, 42 of the respective electron discharge devices I3, I4 is accomplished by connecting a suppressor grid 43, provided in the electron discharge device I1, directly to the automatic volume control circuit 31. In the absence of signal oscillations, the suppresso-r grid 43 is normally maintained at a small positive potential by the connection of cathode 44 provided in the electron discharge device I1 to a point on the voltage divider resistor 45 negative with respect to a grounded point 46 on the resistor. It will be understood that the resistor 45 is connected across a source of. unidirectional potential having a polarity as indicated on the drawing. The potential drop appearing across the voltage divider 45 between the points to which the cathode and ground are respectively connected, produces a current flow in a series circuit comprised by the diode load resistor 3|, the ripple filter resistor 38, and the suppressor grid 43 to'cathode 44 path of the electron discharge device I1. This current flow produces a potential drop in the resistors 3| and 38 having a polarity such that the control grids 4|, 42 of the respective electron discharge devices I3, I4 are biased negatively with respect to their cathodes. By connecting the cathodes 41, 48 of the respective discharge devices I3, I4 to a point 49 on the voltage divider 45 negative with respect to the point of ground connection 46, a normal operating bias is provided for the electron discharge devices I3, I4 such that the -devices I3, I4 operate at full output with a relatively weak signal input. It may be noted that the automatic volume control potential is not impressed upon the control'grid I5 of the electron discharge device I1, the control grid I6 simply being given a normal operating bias determined by the potential drop across that portion of the voltage divider resistor 45 between the point to which the cathode 44 is connected and the negative terminal.

The delayed operation of my automatic volume control arrangement will now be apparent. In the absence of signal oscillations, the bias applied between the control grid and cathode of each of the electron discharge devices I3, I4 is determined by the addition, with proper polarity, of

the constant potential drop across the voltage divider 45 between the points 49 and 46 and that across the resistors 3| and 38. If a weak signal oscillation be now received, the anode 38 of the diode rectier 33 will not pass current since the ungrounded terminal of the diode load resistor is negative and the anode 30 is therefore negative with respect to the diode cathode 32. Since the signal oscillations appearing in the transformer secondary 21 oppose the voltage drop produced across the diode load resistor 3| during alternate half cycles thereof (caused by the suppressor grid 43 to cathode 44 current of the electron discharge device I1), a point is reached as the signal oscillations increase in strength where the diode 33 begins to pass current. As the signal strength increases above this predetermined minimum signal strength, the electron discharge diode rectier 33 passes increasing values of current and a point is soon reached at which the diode current owing through the diode load resistor 3| reduces the potential between the suppressor grid 43 and cathode 44 to zero. Thereafter increasing values of signal strength cause increasing values of potential drop across the diode load resistor 3| and, consequently, increasing negative values of potential in the automatic volume control circuit 31 thereby to decrease more and more the amplification of the radio frequency amplier I8 in the manner explained above.

As the signal oscillations fade or decrease from a relatively strong Value to a relatively weak value, the negative potential appearing in the automatic volume control conductor 31 decreases to a point at which the suppressor grid 43 and cathode 44 are at the same potential. Further decreases in signal strength below this predetermined value cause the suppressor grid 43 to become increasingly positive with respect to its cathode with consequent increases in the value oi space current flowing from the suppressor grid to cathode. Since the resistive drop between the suppressor grid 43 and the cathode 44 is substantially constant, the potential appearing in the automatic volume control circuit 31 remains constant as soon as space current begins to ow between the suppressor grid and cathode. The amplification of the radio frequency amplifier I0 consequently o-perates with a constant normal output for all values of signal strength below the predetermined value at which the suppressor grid 43 becomes positive with respect to its cathode. As the signal oscillations again increase in strength, the amplification factor of the radio frequency amplifier I0 remains constant until the signal oscillations reach a predetermined or threshold strength. Thereafter the amplication factor of the radio frequency amplifier Il) begins to decrease with increasing values in the signal strength to repeat the cycle ofV operation eX- plained above.

This operation is shown graphically in Fig. 2 where the intermediate frequency input to the diode detector in microvolts is plotted as abscissa against the voltage drop across the resistor 3| as ordinates. It will be Vseen from the curves of Fig. 2 that the potential appearing on the automatic volume control conductor 31 remains constant until the intermediate frequency input to the diode detector 33 increases to a strength represented by the point a at which time the voltage drop developed across the resistor 3| is sucient to reduce the potential between the suppressor grid 43 and the cathode 44 of the electron discharge device I1 to zero. Thereafter,

increased Values in the strength of the intermediate frequency oscillations supplied to the diode rectifier 33 result in a substantially linear increase in the automatic volume control potential appearing in the automatic volume control circuit 31 when the graph is plottedv on semi-logarithmic graph paper.

A modication of my invention is shown in Fig. 3 wherein elements corresponding to like elements of Fig. 1 are designated by like reference characters. This arrangement diers from that of Fig. 1 `in that thexed bias for the several electron discharge devices is obtained in a somewhat more simpliiied manner. In this modification, the cathodes 41, 43 of the respective electron discharge devices I3, i4 are connected to ground through a biasing resistor 50 and a bypass condenser 5|. In the absence of potential on the automatic volume control conductor 31, the bias on the control grids 4i, 42 of the devices I3, I4 is determined by the potential drop across the resistor 50 produced by the space current flowing therethrough. The'cathode 44 of the electron discharge device i1 is connected directly to ground and to the lower end of the diode load resistor 3i. The suppressor grid 43 of the device I1 is, as in Fig. l, directly connected to the automatic volume control conductor 31, but in this modification, the suppressor grid is also connected through a high resistor 52, which preferably has a resistance in the order of ten megohms, to the positive terminal 53 of a high voltage direct current supply, the other terminal of which is connected to ground.

The operation of this modification is similar to the operation of the Fig; 1 arrangement with the exception that the unidirectional current flowing through the diode load resistor 3| in the absence of a signal oscillation does not flow in the suppressor grid to cathode path of the device l1. Thus two parallel paths are provided by this arrangement for the current flowing from the high voltage terminal 53 through the resistor 52 to ground. The rst path is comprised by the serially connected 'resistors 38 and 3l, while the second pathv is comprised by the suppressor grid to cathode path of theelectron discharge device i1. 'As in the Fig. l arrangement, space current flows from the suppressor grid 43 to the cathode 44 until such time as a potential drop is built up in the diode load resistor 3l, by the rectification of signal oscillations supplied through the intermediate frequency transformer 25, suicient in magnitude to drive the potential of the suppressor grid 43 to zero with respect to its cathode. When this occurs, the potential drop from the automatic volume control circuit 31 to ground no longer remains constant as it had previously been held by the low resistance drop existing between the suppressor grid and cathode while these elements conducted current. Increasing values of potential drop across the resistor 3l cause the automatic volume control circuit 31 to become increasingly negative and consequently to reduce the gain of the one or more stages of amplication connected to the automatic control circuit.

As soon as the intensity of the received signal oscillations reduces sulciently that the potential drop across the resistor 3l no longer maintains the suppressor grid 43 negative with respect to the cathode 44, a space current begins to flow from the positive terminal 53 of the high voltage source through the resistor 52 and from the suppressor grid 43 to the cathode 44. The

potential drop in the automatic volume control circuit 31, and thereby the gain of the controlled amplier stages, remains constant for signal oscillations having a strength less than this threshold value. It will of course be evident that the minimum strength of signal oscillations to accomplish this delayed operation is determined by a suitable choice in the values of the resistors 3i, 38 and 52. By way of example, the resistor 3l may have a value of 250,000 ohms, the resistor 38 may have a value of 2 megohms, and the resistor 52 may have a value of 10 megohms.

If the value of the potential connected to the terminal 53 is 100 volts, the combination of resistors here specied will result in an 18.4 volt delay in the action of the automatic volume control circuit.

The modications of Figs. 4 and 5 are similar to the modication of Fig. 3 but differ from the latter in their connection of the control grid I6 of the device I1 to the automatic volume control conductor 31. This provides an arrangement in which the automatic volume control potential is supplied to both the suppressor grid 43 and the control grid I6 of the device vl1 whereby an increased response of the device |1.to the automatic volume control potential may be effected.

In the Fig. 4 modification, the control grid i5 lis connected through a biasing battery 54 to the automatic volume control conductor 31. The battery 54, connected into circuit With the polarity indicated on the drawings, has a potential suicient to maintain the controlgrid i5 at the correct negative bias during periods when the suppressor grid 43 is positive, as when no signal oscillations are received in the input circuit of the receiver.

'Ihe battery 54 of Fig. 4 may be replaced as in Fig. 5 with a voltage divider comprised by the resistors 55, 55 which are connected between the automatic volume control conductor 31 and the negative terminal of the voltage divider resistor 45, The relative values of the resistors 55 and 55 are such that the potential dropy across the resistor 55 in the absence of received signal oscillations is sufficiently smaller than that across the left-hand pfortion'5'i of the resistor 45 that the control grid I6 is biased with anormal negative potential. By way of example, if the potential drop across the left-hand portion 51 of the resistor 45 is 18 volts, the resistor 55'may` have a value of 5 megohms and the resistor 56 a value of 1 megohm. With these values, the negative bias on the control grid i6 (in the absence of a signal) The arrangement of Fig. 5 has the advantage that the control grid i5 of the Idevice i1 receives a smaller automatic volume control potential than that supplied to the remaining amplifier stages to which the automatic volume control conducto-r 31 is connected. This is desirable from the standpoint of reducing any tendency of the iii-st stages of radio frequency amplification to overload and thereby to distort when the receiver is tuned to resonance with a signal. If the resistors 55 and 56 have the values suggested, the control grid I6 receives:

Where Esvc=the automatic volume control potential of the conductor 3l Eg: the bias on the grid i6 of the device il.

While I have shown particular embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since many modifications may be made both in the arrangement and the element employed, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention, Y

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In a delayed automatic volume control system, the combination of a source of signal oscillations of varying average intensity, an amplier for said signal oscillations, said amplier including an electron discharge device having a suppressor grid and a cathode, automatic volume control means for maintaining the output of said amplifier substantially constant as said signals vary in average intensity, means for producing a flow of current from said suppressor grid to said cathode in response to a decrease of signal strength below a predetermined value, and means responsive to said ow of current for terminating the control over said amplier of said automatic volume control means.

2. In a delayed automatic volume control system, the combination of a source of signal oscillations of varying average intensity, an amplier for said signal oscillations, said amplier including an electron discharge device having a control electrode and a grid, automatic volume control means connected to control the magnitude of bias on said control electrode for maintaining the output of said amplifier substantially constant as said signals vary in average intensity, and means responsive to the decrease of the bias on said control electrode to a predetermined small value for changing the bias on said grid from negative to positive whereby the control of said last named means is terminated in response to a decrease of signal intensity below a predetermined value.

3. The combination, in a signalling system, of means for receiving signal oscillations of varying average intensity, means including electron discharge devices for amplifying said signal oscillations, one of said devices having a control grid and another having a suppressor grid, automatic volume control means connected to said control grid for maintaining the output from said amplifier substantially constant as said signal oscillations vary in average intensity, a source of unidirectional potential, and means including a connection of said suppressor grid to said control grid and to said source forimpressing a small negative bias on said control grid and a small positive bias on said suppressor grid in response to decrease in the intensity of said signal oscillations below a predetermined value.

4. An automatic amplification control comprising, in combination, means for receiving signal oscillations which are subject to varying average intensity, means including electron disfcharge devices for amplifying said signal oscillations, one of said devices having a control gridV and a cathode and another of said devices having a suppressor grid and a cathode, means for driving saidcontrol grid negatively with respect to the cathode of said rst named device upon an increase in the average intensity of said oscillations above a predetermined value thereby to maintain the output from said amplifying means substantially constant, a source of unidirectional potential, and means including a connection of said suppressor grid to said control grid and to said source for limiting the operation of said last named means while the intensity of said signal oscillations is relatively low, said last named means biasing said suppressor grid either positively or negatively with respect to the cathode of said second named device in response to a change in the average intensity of said signal oscillations to a value respectively below or above said predetermined value.

5. A delayedV automatic volume control for signalling system comprising, in combination, means for receiving signals of varying average intensity, means for amplifying said signals, said last named means including electron discharge devices one of which has a control grid and a cathode and another of which has a grid and a cathode, automatic volume control means including a resistor having a terminal connected to said control grid for maintaining the output of said amplifier substantially constant as said signals vary in average intensity, a connection between said control grid and said second named grid, a source of unidirectional potential, and means including a series discharge path for said source through the resistor of said last named means andthrough thegrid to cathode path of said second named discharge device for terminating the control of said last named means in response to the decrease of said average signal intensity below a predetermined value.

6. The combination, in a signalling system, of a signal amplifier including an electron discharge device having aV control electrode, means for biasing said control electrode increasingly negatively upon increasing values of average signal intensity above a predetermined minimum value thereby to maintain the output from said amp'lier substantially constant, and means to prevent action'of said means during periods of weak signal strength, said last named means comprising in said amplifier a second electron discharge device having a grid connected to said control electrode and having a cathode, and means to transmit current between said grid and said cathode during periods of small average signal intensity below said predetermined minimum value whereby the biasing potential on said vcontrol electrode is controlled during such periods by the grid to cathode impedance of said last named device.

'7. The combination, in a signalling system, of means for receiving signals of varying average intensity, means for amplifying said signals, said last named means including electron discharge devices one of which has a control grid and cathode 'and another of which has a grid and cathode, a resistor, means for developing across said resistor a unidirectional potential having a magnitude which increasesV with increasing valuesY of average signal intensity, means for supplying said potential to said control grid and cathode of said rst named device whereby the output of said amplier is maintained substantially constant as said signals vary in average intensity, and means for delaying the development of potential across said resistor by said last named means until the average intensity of said signals increases above a predetermined minimum value, said last named means including a connection from said control grid to said second named grid, a source of unidirectional potential, and connections forming parallel discharge paths from said source through said resistor and through the grid to cathode path of said last named discharge device.

8. In a delayed automatic volume control system, the combination of a source of signal oscillations of varying average intensity, an electron discharge amplifier of said signal oscillations, said amplifier including a control electrode and a grid, means for controlling the bias on said control electrode to maintain the output of said amplifier substantially constant as said signals vary in average intensity, and means responsive to a decrease in intensity of said signal oscillations below a predetermined value for maintaining the bias on said control electrode substantially constant, said last named means including a source of xed biasing potential, means for controlling the bias on said grid by said rst named means, and means including said last named source for maintaining the bias on said grid more positive by the value of said fixed biasing potential than the bias on said control electrode.

9. The combination, in a delayed automatic volume control system, of a source of signal oscillations of varying average intensity, an amplier for said signal oscillations, said amplier including an electron discharge device having a control grid and an additional grid, means responsive to the average intensity of said signal oscillations for developing a small positive potential in the absence of signal oscillations and increasingly larger values of negative potential with increasingly larger values of the average intensity of said signal oscillations, means for supplying the potential of said last named means to said additional grid, a source of constant biasing potential, and means for supplying with opposing polarities the potential of said last named source and a portion of the potential of said last named means to said control grid to bias said control grid slightly negatively in the absence of signal oscillations.

10. -In a delayed automatic volume control system, the combination of a source of signal impulses, means for amplifying said impulses, said means including an electron discharge device having elements including a grid and a. cathode, an amplification control circuit, means for producing across said circuit a control potential Whose magnitude varies in proportion to the average intensity of said signal impulses, means for supplying said control potential to said rst named means to vary the gain thereof inversely with changes in average signal intensity, and means including a connection of said grid and said cathode across said control circuit for impressing a potential on said grid positive with respect to said cathode in response to decreases in the average signal vintensity below a predetermined value whereby said control potential remains constant until the average signal intensity exceeds said predetermined value.

11. In a delayed automatic volume control system, the combination of a source of signal impulses, means for amplifying said impulses, said means including an electron discharge device having a grid and a cathode, an amplification control circuit, means for producing across said circuit a control potential Whose magnitude varies in proportion to the average intensity of said signal impulses, means for supplying said control potential to said flrst named means to vary the gain thereof inversely with changes in average signal intensity, a source of relatively constant electromotive force, and means including a connection of said grid and said cathode in series with said last named source across said control circuit for impressing a relatively small positive bias on said grid in the absence of signal impulses whereby the gain of said rst named means is maintained substantially unchanged until the average signal intensity exceeds a predetermined value.

12. In a signaling system, the combination of a source of signals of varying average intensity, means for amplifying said signals, said means having a pair of electron paths including a lrst and a second grid, means for developing a control potential Whose magnitude varies in proportion to the average signal intensity, means for supplying said control potential as a bias to each of said grids to control the amplication of said first named means in response to an increase in average signal intensity above a predetermined value, and means including one of said grids for delaying the control of amplication by said last named means until the average signal intensity equals or exceeds said predetermined value.

ROBERT B. DOME.

i CERTIFICATE CE CORRECTION. Patent No. 2,172,160. September 5, 1959.

ROBERT B. 1DOT'LF...

It is hereby `certified that error appears inthe printed specification of the above numbered patent requiring correction as follows: lPage 2, seeond column, line' 6, for "resistor" -read resistor page 5, first Column, line 1, 4for the word "increased" read increasing; page LL, first column, line 15, for "element" read elements; and that the said Letters .Patent should be read with this correction therein that the same may Conform to the record of the case in the Patent Office.

Signed and sealed this 17thd'ay of October, A. D. .1959.

Henry Van Ars'dale (Seal) Acting Commissioner of Patents. 

